Throughout the aircraft industry, it is common to use high strength steel and other fatique sensitive materials in areas of high stress. Normally, bushings are used in the high strength housings as bearings for various other parts that must move with respect to the housing. These bushings, in combination with their required lubricant fitting installations, have caused many failures. In some areas, such as the landing gear and flap actuating linkage, these failures are potentially catastrophic. The bushings heretofore in use each have a sharp chamfer lower end, external lubricant ring and a sharply defined relief adjacent the positioning flange formed adjacent on one end thereof. The sharp edged chamfer tends to score and broach the housing hole into which the bushing is inserted which also causes a reduction in the fatigue life of the assembly. Also, the bushing flange relief, lubricant groove and chamfer reduce the housing bearing area which creates fatigue cracks due to high stress concentrations and galling from the grooves in contact with the housing. Due to the high stress concentrations, a relatively low interference fit is all that can be safely established between the bushing and its housing. Not only is this poor from a fatigue standpoint, but in smaller bushings the low interference can allow the bushings to rotate in the housing which further galls and scratches the housing, reducing its fatigue strength. In extreme instances, the bushings have rotated out of the housing.
Since the prior art bushing has an external lubricant groove, means such as holes must be drilled therethrough at selected locations to provide a passageway for the lubricant to pass to the inner surface of the housing. The groove and the lubricant holes which must extend inwardly therefrom make the prior art bushings relatively expensive to fabricate, and since the lubricant is in direct contact with the shaft running through the bushing only at the lubricant holes, shaft to bushing freeze-up due to lack of lubrication is a constant problem.
It has been traditional to supply lubricant to the lubricant groove by means of a hole with extends through the housing to the groove from some outside surface. At the outer extremity of the housing, a threaded hole has been used into which is inserted standard lubricant fittings which have tapered pipe threads. The tapered pipe threads create high localized stress concentrations in the straight threads of the housing when the fitting is installed and torqued to an adequate level. These standard fittings cause the most severe poblems in high strength aluminums that are notch-sensitive such as 7075-T6 aluminum and they also cause fabrication problems in high strength steels which cannot be threaded after heat treatment. Also, due to the normal geometry high strength parts, lubricant fittings must usually be installed in the short transverse grain direction which increases the chances of stress corrosion cracks.
The use of the prior art bushings and lubrication means have caused a large number of fatigue failures. Therefore, there has been a need for a new type bushing and an associated lubricant means therefor which do not adversely effect the fatigue strength of the high strength part in which they are installed.